Coverage now 95%.  We are getting close.  Still not covered in highlight group: 
 
create_page_highlight:  # Highlight only.
    text.bind('<Double-Button-1>', lambda e: 'break')
    text.bind('<B1-Motion>', lambda e: 'break')
Callbacks are never called. I presume that these mask something.  I am not sure that they are needed.

save_new:  # Same as for keys.
        if not idleConf.userCfg['highlight'].has_section(theme_name): 
Never false.

delete_custom:  # Same as for keys.
        if not item_list: 
            self.custom_theme_on['state'] = DISABLED 
            self.customlist.SetMenu(item_list, '- no custom themes -') 
        else: 
            self.customlist.SetMenu(item_list, item_list[0]) 
Condition never false, else never executed.